Various kinds of fiber optical sensors are available, which generally comprise light intensity-modulated fiber optic sensor, fiber optical grating sensor, interference fiber optical sensor, etc. The latter two are characterized by high sensor sensitivity. However, they have the disadvantages of complexity in apparatus, high cost in operation, etc. during the application, whereby the application of the fiber optical sensors is significantly limited. Especially for the fiber optical sensor with higher sensitivity, for example, interference fiber optical sensor, it may response to the changes in various environment conditions due to its high sensitivity during the process of use. However, when it is applied under the practical conditions, its operational parameters may be affected by the environment factors of temperature, air pressure, vibration, etc. Therefore, when it is used, a lot of measures will have to be taken to prevent and remove the influences of the above environment factors so that the structure of the monitoring apparatus gradually tends complex and the cost for use in operation is greatly increased.
Fiber optical microbend sensor is a light intensity-modulated sensor and is characterized by lower cost, high sensitivity and certain capacity of anti-interference of the environment. It is achieved based on the bend or microbend loss of the optical fiber. The change in the light power is caused by changing the bend degree of the optical fiber.
The principle of the loss of light power is that the bend loss may generate when the optical fiber receives a bend disturbance, generally being a microbend loss and a macrobend loss. Both of the bend losses are caused by the guided-mode coupling of part of the fiber core to the cladding when the optical fiber is bended, which may be calculated according to the theoretical formula of Marcuse as follows:POUT=PIN exp(−γS),                wherein PIN and POUT are the input and output power of the light, respectively, γ is the coefficient of a bend loss, and S is the length of a bend arc. It can be seen that the larger the coefficient of the bend loss γ is, that is, the smaller the bend radius of the optical fiber is, the larger the loss is. However, too small bend radius may cause the significantly reduced lifespan of the optical fiber and influence the service life of the sensor, whereby the bend radius of the optical fiber in practical use is limited. On the other hand, with the same coefficient of the bend loss γ, the attenuation may be increased if the bend arc S is increased. The length of bend arc S may be significantly increased for the purpose of greatly improving dynamic range and precision of the fiber optical attenuator.        
The solution proposed in the CN Patent No. 8710210 achieves a fiber optical stress meter mainly with the microbend loss of optical fiber. However, as the fiber optical stress meter is obtained by two flat plates, which may not be very large, the length of the optical fiber which may be bended is limited, thereby preventing the improvement of dynamic range and precision of such a fiber optical attenuator. In addition, the largest adjusted distance between the two flat plates in relative movement is only several hundreds of micrometers, and the two flat plates must maintain parallel substantially in movement. Therefore, higher demand on the adjusted mechanical structure by such an attenuator not only increases the cost for implement, but also limits the improvement of dynamic range and precision of the fiber optical attenuator.